1. Field of the Invention
This invention relates to computers and especially to a portable battery powered computer which includes communication features such as a wireless modem and paging receiver.
2. Description of the Prior Art
Portable computers are well known, as are personal "communicators" of the type exemplified by the Motorola Envoy. Such portable computing devices are invariably battery powered. Since presently available batteries have very limited storage capabilities, it is important that such portable computing devices (both computers and communicators) limit their power draw. Therefore there is known a wide range of techniques for conserving power in such battery powered devices. These power conservation methods include shutting down portions (various subsystems) of the computer when not in use, as well as putting the computer CPU (the main processor) to "sleep" when its capabilities are not being used.
Many portable computers have a main microprocessor of the type commercially available from Intel and other suppliers called generically the "486 " type. This microprocessor (and some others such as the Intel PENTIUM.TM. processors) includes a suspend/resume feature and system management mode, for power management. A special signal generated for instance by a set of key strokes or pushing a special button on the computer or in some cases closing the top of the computer and thereby actuating a switch, is sent to the microprocessor. This command is called the system management interrupt (SMI) and is documented in the Intel document Pentium Processor User's Manual Vol. 1, 1993, pp. 14-1 to 14-2. The microprocessor, upon receiving the SMI when the microprocessor is operating, executes SMI code in its SM (system management) mode, saves its current register state of operation to SRAM memory, and then enters a suspend (sleep) state in which it is not quite off but draws very little power, and hence is for all practical purposes not operating except for checking for a subsequent SMI.
In response to subsequent receipt of an SMI, the microprocessor resumes operation (executes its RSM instruction) with the same state that was previously saved and does so very quickly. Hence the system management interrupt allows "toggling" between the suspend and resume states, allowing power conservation without significantly impeding use of the computer, i.e. there is no need to reboot and reload application programs.
In some computers the system management interrupt is also generated by a timer; when for instance there has been no keyboard input for a particular amount of time, the SMI is generated, putting the computer into the suspend state until additional keyboard activity occurs.
Another feature available in some portable computers is the so called "instant on" feature. This is somewhat different from the suspend state, in that the computer is maintained in a low power standby state with some of the computer subsystems, e.g. the screen, powered down. Any of the usual processor interrupts then bring the entire computer back to full operation. This standby state typically draws more power than does the suspend state.
Thus there is a continuum of exemplary computer states involving increasing amounts of power consumption. The first state is when the computer is actually off and drawing no power (except perhaps for time purposes or to maintain certain data in memory); the next state is the suspend state supported by the SMI; the next state is the standby state which corresponds to the instant on feature; and the last state is the full on state in which all computer subsystems are fully operational.
However neither the instant on feature nor the manually generated SMI resulting in the suspend state are suitable for a portable computer which also includes communication features. Such communication features, while generally not part of portable computers, have been available in communicators. These communicators typically have included only modest computing capabilities but also include for instance a conventional wired modem for connection to telephone lines, a wireless modem for either facsimile, packet data, and/or telephone communications, and a pager receiver (an integrated pager).
Since a purpose of such communicators is to be able to receive messages from an external source at all times, typically via the pager or the wireless modem, the communicator must be at least partly powered up at all times. Thus it is maintained in a low power state (not off) even when the user is not actively using it, in order to receive messages by paging or radio modem transmission. This (relatively) low power state in fact requires a relatively high power drain because significant portions of the communicator must be on at all times in order to be able to receive the messages. Hence these communicators suffer from needing frequent battery changes and/or recharges.